I went to school with a Marybeth and a MaryJane, each of them had middle names. Each of them only had three initials, to my understanding, since there was no space between Mary and the respective name.
My cousin John Paul doesn't have a middle name however since he has a double barrel name. Another cousin is Shelby Ann where her first name is Shelby and her middle is Ann and she's just referred to with both names.
I would think John Mark Andrew Porter would be JAP however you could also use JMAP is you wanted.

Thanks! Our daughter is Nora Kate, just a first and middle. Most a lot of the time we call her both, as if she has a double first name. I think if we ever had another boy, we might just give him the first and middle John Mark and use it as a double first.